ISOTOPIC ZONING AND ORIGIN OF THE AQUIFERS IN THE DISCHARGE AREA OF THE GEOTHERMAL FIELDS OF AHUACHAPAN-CHIPILAPA, EL-SALVADOR

The northern discharge areas of the Ahuachapan and Chipilapa geotherma l fields can be subdivided into four different zones based on their st ructural position, and the isotopic and chemical composition of their waters. In general, the contribution of geothermal waters from these t wo fields was estimated to be less than 10%. Elevation effects are of little importance, whereas a slight trend towards higher isotopic valu es with increasing water temperatures may exist.The NNW-SSE-trending E scalante and Agua Caliente faults represent lateral groundwater barrie rs, and provide vertical conduits for the ascending geothermal waters. The western discharge areas seem to be more influenced by the Ahuacha pan geothermal field, whereas those to the east are more influenced by the Chipilapa field. Groundwaters in the Northern Plain are mainly fr om shallow northward-flowing aquifers. These waters show temperature e ffects,mixing with geothermal waters and are affected by the geology o f the area. However, none of these factors alone can explain the isoto pic variations observed in the waters of the northern discharge areas. (C) 1997 CNR. Published by Elsevier Science Ltd.